1. Field of the Invention
The invention relates to a drive system for a rotating electric machine which functions as a running drive source for a vehicle. More particularly, the invention relates to a discharge technique of a smoothing capacitor provided in a drive system.
2. Description of Related Art
Motor-driven vehicles having a rotating electric machine installed therein as a running drive source, for example, hybrid automobiles and electric automobiles are widely available. Such motor-driven vehicle is usually provided with an inverter supplying alternating current (AC) power to the rotating electric machine, a battery connected to the inverter and a smoothing capacitor that smoothes a voltage between the inverter terminals. In such a vehicle, it is desirable that the switching element of the inverter be switched and the electric charge of the smoothing capacitor be discharged in the event of vehicle collision. In order to ensure such a discharge, Japanese Patent Application Publication No. 2005-020952 (JP-2005-020952 A) discloses a technique of performing switching control of the inverter with the object of discharging the electric charge of the smoothing capacitor when a vehicle collision is predicted. With such a technique, the inverter can be driven to perform the discharge and therefore the electric charge of the capacitor can be reliably discharged before the drive circuit of the inverter is crushed by the collision.
However, the problem associated with such a technique is that where a rotating electric machine rotates when such a discharge treatment is performed, a back electromotive force is generated and the discharge becomes impossible or requires a significant time. In particular, when a drive shaft comes out as a result of vehicle collision or when the vehicle turns over, even if the vehicle stops, the motor continues rotating, thereby easily causing the aforementioned problem.
Japanese Patent Application Publication No. 2005-094883 (JP-2005-094883 A) and Japanese Patent Application Publication No. 2006-020450 (JP-2006-020450 A) disclose techniques of, upon detection of vehicle collision, switching off a switching element on the upper arm side of the inverter and switching on a switching element on the lower arm side, thereby generating a brake force for the vehicle, and then switching on all of the switching elements to discharge the electric charge remaining in the smoothing capacitor. By so generating a brake force and reducing the revolution speed of the rotating electric machine prior to the discharge, it is possible to reduce further the time from the collision to the completion of the discharge.
However, with the techniques described in JP-2005-094883 A and JP-2006-020450 A, the revolution speed of the rotating electric machine cannot be reduced efficiently. As a result, a significant time is required to the completion of the discharge even with the techniques described in JP-2005-094883 A and JP-2006-020450 A.